The present invention relates to an apparatus for stabilizing and supporting a zone refined crystal rod from the action of a seed rod on a melt of a crystalline source rod, and more particularly to the advancements of said apparatus which permit the processing of larger diameter rods, and longer rods, without unduly increasing the overall hazard of fracture of the crystal neck or spill of the melt zone.
Conventional apparatus for zone refining includes an induction heating chamber. A rod holder for holding a crystalline source rod to be refined is introduced into the chamber top. A seed holder for holding a semiconductor seed crystal comes into the chamber from the bottom wherein the seed crystal is held. The chamber has a door with a window, the window being necessary to permit the operator to watch the zone refining operation especially during the initial stage when the seed crystal is fused to the molten end of the semiconductor source rod. In a typical apparatus, a semiconductor source rod on the order of 110 centimeters in length can be refined. Such a source rod is attached to the rod holder at its upper end, and a small diameter seed crystal of about 4 to 6 millimeters is attached to the seed holder. A heavy RF induction heating coil of suitable design is positioned near the middle of the induction heating chamber. The chamber is then either evacuated or filled with inert gas, such as argon. The source rod holder and source rod are moved down from the top of the chamber with the free end of the rod approaching the RF induction heating coil. The RF coil inductively heats and melts the bottom or free end of the semiconductor source rod until a molten droplet is formed. At that point, the seed rod moves up to the molten end of the source rod within the heating zone of the RF coil. The seed crystal fuses and is pulled away to create a neck and then a taper at the molten end of the semiconductor source rod. Thereafter, the zone of the melt is moved up the semiconductor source rod by moving both the rod and the seed crystal downward. Relative movement between the seed rod holder and source rod holder controls the diameter of the refined crystal semiconductor rod, and in addition, the source rod holder and seed rod holder can be independently rotated as both move downwardly with respect to the induction heating chamber.
Until recently the minimum height for conventional zone refining apparatus was at least four and often five times the length of the crystalline source rod to be refined. The induction heating chamber itself had to be twice as long as the rod to accommodate the flow of movement of the source rod through the RF coil. The fully extended positions of the source rod holder and the seed rod holder were each equal to the length of the rod at a minimum. Since the initial stages of zone refining operation are very time consuming, and require constant monitoring by highly skilled operators, a determined effort has been made by the industry to process larger diameters and longer rods. Recent developments such as the use of bellows ended chambers wherein the chambers may be extended in length have permitted longer source rods and longer refined crystal rod products. For example, zone refining apparatus is provided with an induction heating chamber the longitudinal dimension which is not determined by the length of the rod to be refined, but instead is limited only by considerations of space for the RF coil and related apparatus which must be mounted within the chamber, viewing space for the operator, and the heating effect on the structure above and below the chamber. More specifically, the semiconductor rod is moved and the RF coil is stationary, just as in a conventional apparatus. However, the semiconductor rod extends above the induction heating chamber initially, and below the chamber at the conclusion of the refining process. An upper metal bellows extends from the top of the rod holder to the top of the chamber. A similar lower metal bellows extends from the bottom of the seed holder to the bottom of the chamber. The two bellows thus keep the working space both gas and vacuum tight. A telescoping group of cylinders are placed inside the lower bellows to protect the lower bellows from any molten semiconductor which may spill or drop from the melt zone.
These apparatus improvements have permitted the ever increasing size of zone refined crystal semiconductor rods not only in diameter but in length as well. Enlarged crystal rods create new problems such as increased oscillations due to the thin necking or bridging piece between the crystal source rod and the seed rod. For example, in the case of semiconductor silicon, this seed rod must be pulled down to a neck 2 to 3 millimeters in diameter and 2 to 4 centimeters in length to obtain dislocation-free crystal structure. These oscillations appear to cause a development of dislocations and other irregularities to be found in a single crystal or monocrystal zone refined rods. These oscillations also frequently cause a dripping or spilling over of the molten material from the melt zone or even a breakage of the neck between the seed crystal and the refined semiconductor rod which of course causes an interruption of the zone refining process.
Increasing refined crystal rod diameters from 25 to 75, or 100 millimeters and greater; and crystal lengths increasing from 3 to 6 fold create tremendous stress and strain on the small neck which has been expected to maintain the weight of these ever larger crystals. Provided that the weight of the enlarged refined crystal rod remains exactly centered over the neck, the neck is capable of maintaining the weight. In practice, this very seldom happens, since normally orbiting or oscillating motions occur, in which the center of the crystal at the plane of growth no longer coincides with the zone refiner axis passing through the center of the seed rod chuck, the working coil, and the crystal seed rod. Since the crystal is rotating during growth, the center describes an orbital motion about the zone refiner axis. The orbit will become larger as the crystal length increases. At present, with crystal lengths of about 50 centimeters the out of alignment orbit becomes excessive, and either the seed breaks or the melt supported by the growing rods pulls away from the melt supplied by the source rod and spills down the side of the growing crystal. In either case, not only must the run be terminated, but the already grown portion may fracture due to thermal shock.
It is therefore desirable to provide an apparatus and a method which will support and stabilize the lower portion of a grown or fabricated crystal and can restrain its lateral motion about the zone refiner axis to an acceptable low value. This supporting and stabilization must be done without contaminating the environment of the growing crystal; without causing thermal shock at points of contact between the crystal and the apparatus, and without upsetting the growth conditions at the liquid-solid interface. These conditions are necessary in order to produce single crystal semiconductor material which is free of dislocations.